In order to investigate the heat stability of catalase in immature and mature erythrocytes of normal, acatalasemic and heterozygous hypocatalasemic mics, phenylhydrazine hydrochloride (PHZ) was administered to these mice at a dose of 40 μg/g of body weight subcutaneously daily for 3 days Anemic blood rich in reticulocytes was obtained 2 days after final injection. Thermostability of catalase in the non-anemic and anemic bloods was compared and the following results were obtained. 1. Specific catalase activity (catalase activity/amount of hemoglobin; PU/g Hb) in anemic blood from acatalasemic mice was significantly higher than that in non-anemic blood. Specific catalase activity in anemic blood from normal mice was a little higher than that in non-anemic blood from normal mice. 2. The residual catalase activity in the hemolysate after heat treatment was compared with that from before treatment in which hemolysate containing catalase was incubated at 42, 44, 46, 48 and 50 °C for 10 minutes in a water bath. At any temperature, catalase activity in immature red blood cells of normal and acatalasemic mice was more stable with heat than that inmature red blood cells. 3. In mormal mice, the T(50) (the temperature at which 50 % of the catalase activity remained after heat treatment) in anemic normal blood was statistically higher than that in non-anemic blood. In the acatalasemic mice, the T(12.5) (the temperature at which 12.5 % of the catalase activity remained after heat treatment) in anemic blood was statistically higher than that in non-anemic blood.